Rotor blades of wind turbines have become increasingly susceptible to lightning strikes, as the rated electric power of the wind turbines and the dimensions of the rotor blades have increased. A lightning strike may adversely impact the condition of a wind turbine. For example, the lightning strike may damage rotor blades of the wind turbine by carbonizing or puncturing the surface of the rotor blades. Furthermore, the lightning strike and its associated intense electric field activity may form electric arcs inside the rotor blades that may increase the temperature inside the rotor blades and damage the rotor blades. The damage may deteriorate the functionality and/or lifetime of the rotor blade, and may further provide a preferred path to a further lightning strike.
Conventional systems have been proposed to protect rotor blades of wind turbines from adverse effects of lightning strikes. In one such system, for example, a rotor blade of a wind turbine is equipped with one or more metal lightning receptors, so-called air termination systems, located on the outer surface of the rotor blade, the metal receptors of the rotor blade are electrically coupled to a respective down-conductor that is installed within the interior of the rotor blade, and the down-conductor of the rotor blade is coupled to an earthed conductor that is coupled to the earth (ground). In the event of a lightning strike on a metal receptor, a lightning current is received by the metal receptor, and flows to the down-conductor. Due to the electrical coupling of the down-conductor to the earthed conductor and its low impedance, the lightning current flows from the down-conductor into the earthed conductor and thereafter flows into the earth.
One challenge for metal receptor based systems is that the lightning may not always strike the metal receptors. For example, the lightning may strike the surface of the rotor blade that is generally made of a non-conductive composite material. The strike of the lightning on the surface may puncture the surface of the rotor blade. The punctured surface then creates a path for the initial or a further lightning strike to enter into an internal cavity of the rotor blade. Before, during and, after lightning strikes into the internal cavity, an electric field may be generated inside the internal cavity and ionize air inside the internal cavity. The ionization of the air may result in formation of electric arcs or partial discharges in the form of streamers inside the internal cavity between a surface of the rotor blade and the down-conductor. The electric arcs may further be formed in the form of leaders along an inside surface or an inside layer of the rotor blade and generate pressure shock waves that may destroy the surface and other portions of the rotor blade. Furthermore, electric arcs tend to increase the temperature inside the rotor blade, thereby causing additional damage to the rotor blade.
Another possible cause of electric arc formation in rotor blades may be electrostatic charging of the rotor blades. The electrostatic charging may occur when there is an increased activity of storm clouds and stormy weather, for example.
Therefore, it would be advantageous to provide improved systems to detect lightning strikes and to determine the presence of electric arcs in rotor blades of wind turbines.